Melt the butter in a 1-quart saucepan and then stir in the flour with a whisk. Cook for 1 minute, mixture should be bubbling, but not browning
Slowly whisk in the milk. Bring mixture to a boil, whisking constantly to prevent lumps from forming. When thickened, reduce heat to low and stir in Gruyere and 3 tablespoons of Parmesan. After cheese is melted, remove from heat, mix in nutmeg, and season to taste with salt and pepper
Preheat broiler on low. Place bread in casserole dish and toast under broiler until golden brown, about 2 minutes and remove from oven
Divide turkey among the 4 pieces of toasted bread. Top each piece with a tomato slice and then spoon cheese sauce over the tomato and turkey. Sprinkle with remaining Parmesan
Place casserole dish under the broiler and broil until lightly golden and the sauce is starting to bubble, about 2-3 minutes. Remove from broiler and finish each hot brown with 2 pieces of bacon. Serve immediately
